You can upload both Windows 8 and Windows 8.1 versions of your app to the store and users will download the most appropriate one. Windows 8 users will get the Windows 8 version. Windows 8.1 users will get the Windows 8.1 version if it exists, otherwise they'll
get the Windows 8 version.

See the
Windows 8.1 Feature Guide for information on what's new in Windows 8.1 and the
Migration Guide for details on updating your Windows 8 app. The migration guide will automatically be offered when you upgrade your project in Visual Studio 2013.